You are using the ansible-vault command. You are not being prompted for a password as the documentation suggests you would be. You want to use ansible-vault create or ansible-vault edit and be prompted for a password. How can you specify a password manually? How can someone on a different Ansible server decrypt your encrypted files?
Possible solution #1
Modify the ansible.cfg file. Comment out a stanza like this:
# vault_password_file = ~/.vault_pass.txt
If there is no leading pound "#" mark, add one to comment it out.
Possible solution #2
Upgrade or downgrade Ansible from 2.4. There is a documented bug about this.
Possible solution #3
When you run ansible-vault, use this flag: --ask-vault-pass
Here is an example: ansible-vault create contint.yml --ask-vault-pass